Energy storage is important for sustainable energy supply from sources such as wind and the sun.
Energy stored as heat and retrieved as mechanical energy has been around ever since the liquid air car (U.S. Pat. No. 657,124 from 1899) and the Ruths' steam accumulator (U.S. Pat. No. 1,294,714 from 1919).
Saipem-Ruer (U.S. Pat. No. 8,443,605), Isentropic Ltd. (U.S. Pat. No. 8,826,664), and Siemens (US Patent Application Publication No. 2015/0059342) use a Brayton cycle with solid hot and cold storage media. The disadvantages of this approach include pressure losses when the working gas traverses through the heat storage and the expensive high pressure storage tanks. There are also difficulties with a uniform stable moving temperature gradient in the storage tanks.
Pumped heat storage liquid has an advantage with cheaper low pressure tanks for the high temperature storage liquid. The Andasol in Spain and Solana in Arizona use mirror-based solar energy collectors, salt heat storage and steam turbines. The efficiency of the Andasol steam cycle is 35%, and 76 kg of salt per stored electric kWh (13.5 Wh/kg).
ABB (EP Patent No. 2,778,406) concerns a reversible CO2 Brayton cycle, working at or near ambient temperature, using pumped heat storage and high temperature pumped heat storage. The heat storage is common with a solar collector system.
Alliance (US Patent Application Publication No. 2012/0216536) concerns a carbon dioxide Brayton cycle for solar heat, which in one embodiment, is combined with a transcritical cycle.
This “Discussion of the Background” section is provided for background information only. The statements in this “Discussion of the Background” are not an admission that the subject matter disclosed in this “Discussion of the Background” section constitutes prior art to the present disclosure, and no part of this “Discussion of the Background” section may be used as an admission that any part of this application, including this “Discussion of the Background” section, constitutes prior art to the present disclosure.